The witness and other nearby Iraqis fled their cars and took cover behind cement barriers. They watched as two small helicopters – Blackwater USA's signature "little birds"– swarmed the area and began shooting at the street.

An Iraqi Ministry of Defense report says that 20 Iraqis were killed in the incident, including a mother and child. US officials say they are conducting their own investigation.

"It's a tragic incident, but we don't know what we don't know at this point," says one American officer with an interest in the security situation. "I see a commitment to get to the bottom of what happened and take action based on the findings. I do understand how Iraqis could be angered by what they've heard thus far."

In a sign of how serious the situation has become, the US Embassy in Baghdad suspended diplomatic travel outside the protected Green Zone on Tuesday. Blackwater USA, based in Moyock, N.C., is one of three firms employed by the State Department to provide protection for US missions in Iraq. The others are Dyncorp and Triple Canopy, both based in Washington's Virginia suburbs.

In the case of the latest incident, Blackwater spokeswoman Anne Tyrrell said in a statement Monday that the company's contractors "acted lawfully and appropriately.... Blackwater regrets any loss of life but this convoy was violently attacked by armed insurgents, not civilians, and our people did their job to defend human life."

Iraqis have long bristled at the presence of the private guards, who they claim are little more than mercenaries with little respect for Iraqi lives and less discipline than uniformed US troops.

An Iraqi police officer who works in Karada, a mixed sectarian neighborhood in eastern Baghdad, says the foreign private security firms act out of their own interests as they jet through the city and seem to pay little heed to the dangers they pose to average citizens on the street.

The officer says employees of the firms use overly aggressive tactics, crashing into cars and disobeying traffic laws and often rolling over gardens and hitting trees – and never stopping.

He says he once tried to help an Iraqi driver who was gravely wounded by private security guards even though he had tried to get out of their way. "They are bad," he says.

A 2004 regulation, promulgated by the US occupation officials who then ran Iraq, granted US private security contractors full immunity from prosecution under Iraqi law.

Technically, they could be prosecuted in US courts for misdeeds in Iraq under certain circumstances, according to a July Congressional Research Service (CRS) report on the subject.

However, their prosecution in US military courts could be subject to constitutional challenge, notes CRS. And there are practical limits – such as the difficulty of collecting evidence – on the ability of US civilian courts to handle such cases.

"It is possible that some contractors may remain outside the jurisdiction of US courts, civil or military, for improper conduct in Iraq," concludes CRS.

This legal gray area stems in part from the fact that the Iraq conflict represents the first time the US has depended on private contractors to provide widespread security services in a hostile environment.

There are an estimated 20,000 to 30,000 such guards-for-hire in Iraq – a small fraction of the 182,000 civilian contractors employed by the US for everything from food service jobs to trash collection.

Most of the security personnel work for the Department of Defense or US intelligence agencies. About 1,400 are employed by the Department of State, according to US government figures.

Of these, some 1,000 are Blackwater employees. About three-quarters of the Blackwater personnel are US citizens, with the rest Iraqis and third-country nationals.

In recent Senate testimony, US Ambassador to Iraq Ryan Crocker said, "There is simply no way at all that the State Department's Bureau of Diplomatic Security could ever have enough full-time personnel to staff the security function in Iraq. There is no alternative except through contracts."

Blackwater was founded by former Navy SEAL Erik Prince. Other company leaders are also former military special operations veterans. The company describes itself as the most comprehensive such firm in the world.

It detractors say it may be the most aggressive, as well. Both feared and revered, Blackwater has a certain tough image in the Wild West atmosphere of today's Iraq. Security personnel and others who work in the country all have stories to tell about Blackwater contractors, known for their devil-may-care attitude on the roads and aggressive tactics.

Blackwater helicopters swirl through the skies like insects. Distinctive for their spherical glass canopies, and their persistent whine, they inadvertently announce that an official entourage is racing along, somewhere down below.

At least 15 Blackwater employee have been killed in Iraq, in fire fights and ambushes as well as in helicopter crashes. Family members of four Blackwater employees who were murdered in Fallujah in March 2004, an incident that led to the full-scale US assault on that city, allege the company sent the men into dangerous territory without adequate backup. In testimony before the US Congress last February, family members of the men family members of the men said: "Private military contractors like Blackwater operate outside the military's chain of command and can literally do whatever they please without any liability or accountability from the US government."

According to the four family's statement, the men killed in Fallujah had been promised armored vehicles, six man teams and extensive briefings with maps and intelligence information before conducting missions in Iraq. In the Fallujah incident, none of that was provided, the families said.

"In fact, when Scott Helvenston [one of the murdered Blackwater employees] asked for a map of the route, he was told "it's a little late for a map now."

The firm has also been involved in some notorious past incidents, including one last Christmas, in which an inebriated off-duty Blackwater employee shot and killed a guard working for Iraqi Vice President Adel Abdul Mahdi inside the Green Zone.

At time of writing, the Iraqi government had suspended Blackwater operations within the country.

If that ban is made permanent, "they will sell their client list and employees to some other company," says John Pike, director of GlobalSecurity.org. "The other contractors will try to get a much better sense of what the rules are and who makes and enforces them."

That the contractors are not subject to Iraqi control may well be an untenable situation, note experts. But it would be difficult to ban all of them outright, considering their importance to the US.

"Nobody is going to be able to throw the contractors out of there," says David Isenberg of the British-American Security Information Council. "They're the American Express card of the American military. The military doesn't leave home without them, because it can't."